Scientists continue to search for clues in dolphin die-off

More than 228 dead dolphins have washed ashore between Virginia and New Jersey this summer, and the cause is still a mystery. NOAA is monitoring the situation, saying it could be anything from disease or human interactions with fishing gear. Experts say the carcasses pose no health risk to humans.

(CBS News) Scientists continue to search for answers surrounding the dolphin die-off occurring along the East Coast.

As of August 12, 228 Atlantic bottlenose dolphins have turned up dead along mid-Atlantic beaches, with the highest concentrations in New Jersey and Virginia. The largest die-off in nearly 25 years, the National Oceanic and Atmospheric Administration (NOAA) classified it as "unusual mortality event" (UME).

"We generally don't see two dolphins reported in one day, and we started seeing that," Kimberly Dunham, director of the Riverhead Foundation Rescue Program in New York, told CBS New York. In recent years, the annual tally has rarely surpassed 100 for the Virginia to New Jersey region.

UMEs are defined as "significant die-off of any marine mammal population" that "demands immediate response." Before scientists can properly respond, they need to understand what is causing the die-off. Signs point towards morbilivirus, a form of measles that killed more than 740 bottlenose dolphins in 1987. Several dolphins have already tested positive for morbilivirus this year.

NOAA National Marine Fisheries spokeswoman Maggie Mooney-Seus told CBSNews.com that tissue and blood samples are being studied, but the results will take a while.

"We've only had a few results from animals in New Jersey that tested positive for morbilivirus," she said. "But it's not clear if that was a cause of death, or if it was compromised immune system."

Other potential factors include pollutants, toxins, and malnutrition. There is also a possibility of a network of factors. Higher water temperatures, which are attributed to climate change, allow diseases to transfer more rapidly within species. Exposures to toxins such as mercury weaken the dolphins' immune systems, leaving them more susceptible to disease.

"Based on the rapid increase in strandings over the last two weeks and the geographic extent of these mortalities, an infectious pathogen is at the top of the list of potential causes for this UME, but all potential causes of these mortalities will be evaluated," NOAA explained on its website.

This is not the first UME of the year. More than 600 manatees have died in Florida, while 362 sea lions are dead in California. Dolphin UMEs are being reported in Italy and Australia. Last year, more than 900 dolphins and 4,000 pelicans died in Peru. While no one likes to hear about mass animal die-off, scientists are particularly concerned to see it happening in the dolphin population.

"Dolphins are at the front lines of ocean health," Matt Huelsenbeck, a marine scientists at the non-profit Oceana, explained to Quartz. Dolphins are "the top predators -- everything trickles [through them] ... They're more important than a lot of research buoys in terms of what's going on."